1. Field of the Invention
The present invention relates to a mandrel conveying device for a multi-stand continuous tube rolling mill operating with a mandrel.
2. State of the Art
Longitudinal multi-stand rolling mills of the prior art, operating with mandrel, may be grouped conventionally into various types, according to their architecture, with particular regards to the rolling speed control and to the speed and position of the mandrel within the tube.
Continuous floating (i.e. free) mandrel rolling mills are those in which the mandrel can move freely inside the tube during the rolling passage through the multi-stand rolling mill according to the friction forces which are generated between mandrel and inner wall of the tube. The mandrel thus gradually accelerates as the rolling stands bite in sequence. The mandrel is removed from the tube at the end of the rolling operation off the rolling line, or in all cases when the tail of the tube has exited from the last rolling stand, and thus when the free mandrel has assumed the same advancement speed as the tube. Very short cycle times, and thus high productivity, are obtained, e.g. up to a production of 4-5 pieces per minute, with this type of rolling mills.
On the other hand, this type of rolling mill is subject to various drawbacks. The acceleration of the mandrel causes compression states in the tube which are detrimental to the dimensional quality and tubes faultiness because the groove delimited by the rolls is overfilled in the first stands and underfilled in the finishing stands at the end of the rolling mill. Problems of rolling stability and of products with excessive tolerances are thus found. Furthermore, the tube cooling is not uniform along the entire length of the tube because the head part of the tube where the mandrel no longer reaches, immediately after the first step of rolling, remains hot for a longer time, while the part behind, in which the mandrel is still inserted while rolling proceeds, is partially cooled by the mandrel itself with which it is in contact. In these rolling mills, it is normally necessary to provide a downstream heating oven to uniform the tube temperature before the final rolling for calibrating or further reducing the tube diameter.
A second type of rolling mill is that known as semi-floating mandrel rolling mill, in which the mandrel is withheld and advanced, slower than the tube, at the technologically favourable speed during rolling. At the end of the rolling operation, after the tail of the tube has left the last rolling stand, the mandrel is released from the retaining device while remaining within the tube itself and following it while it is moved away from the rolling line. The mandrel is removed from the tube off the rolling line, or however when the tail of the tube has exited from the last rolling stand, and thus when the free mandrel has assumed the same advancement speed as the tube. Short cycle times, and consequently high productivity rates, are obtained in rolling mills of this type, for example: 3-4 tubes per minute. On the contrary, equivalent drawbacks to those of rolling mills of the previous type are found with regards to lack of temperature uniformity along the tube.
A third type of rolling mill is called retained mandrel rolling mill and is characterized by the presence of a rack and pinion mandrel retaining device. At the end of a tube rolling operation, when the tail of the tube leaves the last stand of the rolling mill, the tube has already been previously engaged by its head portion downstream of the rolling mill by means of an extractor device, which grips onto the outer surface of the tube. The extractor device, which is generally made in the form of a particular sequence of roll rolling stands, drags the tube forwards in the same rolling direction, while the retaining system locks the mandrel to make it run inside the tube, and pulls it backwards towards the inlet side of the rolling mill from where it is then unloaded and reintroduced in the classic mandrel conveying cycle. The extractor device or mill also has the function of reducing the outer diameter of the tube by rolling it further without mandrel inside while it is removed. The cycle times of this type of rolling mill are longer, and thus productivity is lower than the previously described types: 2 tubes per minute can generally be rolled.
In traditional rolling in retained mandrel plants, the mandrel advances during the step of rolling at controlled speed, also called retaining speed, directed in the same direction of motion as the tube from the inlet to the outlet of the multi-stand rolling mill during the entire rolling cycle.
In rolling processes implemented using this type of rolling plant, the mandrel is normally inserted into the pierced shell at the beginning of each rolling cycle, starting from the tail in direction of the head of the pierced shell itself, with motion in the same sense as the direction of rolling of the tube.
This first operation may occur in line with the rolling axis, and this is known as in-line insertion, or off line, and this case is known as pre-insertion, the pre-insertion of the mandrel in the pierced shell being used to reduce the travel of the mandrel retaining devices, and thus to reduce the cycle time of the rolling mill itself, increasing productivity. A limitation of this technology is thus its low productivity, in particular for rolling mills used for rolling small and medium sized tubes, e.g. those which have a nominal diameter smaller than or equal to 7″ (177.8 mm).
Another type of rolling mill is the one named retained mandrel type with extractor and tube release at the end of rolling, with passage of the mandrel through the extractor. The rolling process carried out in this type of rolling mill includes that, at the end of the tube rolling operation, the mandrel is immobilized in the specific retaining device, while the tube is removed from the mandrel by means of the extractor device by pulling it along the rolling line. After the tube has passed all through the extractor device, the mandrel is then released from the retaining device, conveyed forward by pressing rolls along the rolling line and is passed through the extractor device immediately after the tube and is finally unloaded downstream of the extractor itself to follow the circuit provided for the mandrel reuse. Relatively short cycle times are obtained in these rolling mills: 2.5 tubes per minute.
A disadvantage of the latter type of plant is that the process includes conveying the still very hot mandrel by means of pressing rolls with the risk of damaging the mandrel surface. In this type of process, the mandrel retaining device during the step of rolling, normally of the rack type, must include a releasing device which works in cycle and is adapted to release the mandrel itself after removing the tube.
In order to implement the rolling process in a rolling plant with retained mandrel, the passage of the mandrel through the extractor mill requires the latter to be made with a stand which can open and close quickly to allow the passage of the rolled tube first and then the mandrel at each rolling cycle, given the high speeds at which pierced shells, tubes and mandrels move along the rolling line. If the operating accuracy of the extractor device is not guaranteed there may be the risk of misalignment of the edges of two adjacent rolls with the consequent longitudinal marking of the rolled tube.
The processes with retained mandrel type rolling plants are in all cases advantageous with regards to tube quality which can be obtained and the thermal conditions in which the tube leaves the rolling mill; indeed only in this type of rolling mill it is possible to include the calibration of the final diameter of the tube also without intermediate heating.
In order to guarantee also an efficient rolling process, either of the retained or semi-floating mandrel type, it is important to use a mandrel retaining device which guarantees mandrel speed stability during the rolling cycle, is robust and offers the possibility of hooking and releasing the mandrel itself, said possibility not being offered by a chain and sprocket system, the latter being very common today. Indeed, in the case of rolling plant with semi-floating or retained mandrel, a retaining device with chain wrapped on sprockets and provided with hooking carriers is disadvantageous due to the premature wear of its components, the noise and the elongation that the chain undergoes over time. In order to avoid such drawbacks of the chain system, in-cycle hooking and releasing systems are used in some known plants of the controlled speed retained mandrel type to implement short cycle time rolling methods. In all cases, these systems do not work centred with the mandrel traction axis, and thus add problems related to flexion loads acting on the hooking/releasing systems.
A rolling mill and retained mandrel rolling process thereof is disclosed in document WO2011/000819, in which, after extracting the tube while the mandrel is still retained and the tube is conveyed and rolled through the extractor device without the mandrel being inside any longer, the mandrel is evacuated from the rolling line downstream of the rolling mill and laterally with respect to the rolling line.
In the known retained mandrel plants, described above, it is however difficult to make short tubes because the latter are shorter than the distance between the axis of the last stand of the multi-stand rolling mill and the first stand of the extractor.
The market requires rolling plants which allow greater final product flexibility, i.e. which are capable of rolling tubes of different lengths, with replacement operations concerning a minimum number of plant components, which allow to reduce the rolling cycle time of the tubes and to increase the global productivity of the plant, which increase the finished tube quality or which at least do not penalise it, which have a more rational structure of the plant itself, reducing manufacturing and management costs of the plant.